The head-up display systems, whether worn or not, make it possible to display in particular a “symbology” conforming to the outside world, that is to say a set of symbols whose position in front of the eye of the pilot allows for a superimposition with the corresponding elements in the outside world. It may be for example a speed vector, a target on the ground or in the air, a synthetic representation of the terrain or even a sensor image.
This conformal display requires knowledge of the position and the attitude of the aircraft and, for the head-worn display devices, the attitude of the display relative to a fixed reference frame linked to the aircraft. These various positions and attitudes are supplied by the avionics systems for those of the aircraft, and by the posture detection device DDP for those of the display.
For example and in particular, the avionics systems for supplying the position and attitude of an aircraft can be, respectively:                a global positioning device of GPS (global positioning system) type; and        an inertial reference system IRS based on gyroscopes and accelerometers of MEMS (micro electro mechanical systems) type or laser gyroscope type, or an attitude and heading reference system AHRS.        
As is known, a harmonization of the head-worn display system is performed on installation of the display system, in a cockpit, in order to compute the corrections of angles to be made to switch from the display reference frame to the aircraft reference frame, and in order to obtain a conformal head-up display.
Now, some head-worn display devices these days have a certain mobility between the display device or display and the worn part of the posture detection system DDP, because of an absence of mechanical rigidity between these two elements, i.e. the display and the mobile worn part of the DDP, for example when there is a device for tilting the display alone outside of the field of view of the operator. There is then a need, when the display is once again tilted into the field of view of the operator, to once again proceed with a harmonization in order to compute new corrections of angle to be made to the head once the head-up display is installed and thus be able to display a conformal symbology in the display device worn on the head.
In order to make it possible and to facilitate this relatively frequent need for reharmonization, it is known practice to install a dedicated instrument on board the aircraft, called boresight reference unit or boresight reticle unit BRU.
The boresight reference unit BRU, installed in the cockpit facing the head of the operator displays a collimated symbol with an orientation that is fixed and known to the head-up system.
Each time there is a need to realign the conformal symbology, i.e. for reharmonization, the operator aligns a symbol displayed in his or her head-up display with the collimated symbol of the boresight reference unit BRU.
When the symbol displayed in the head-up display, i.e. the display is aligned on the collimated symbol, the detection-device output harmonization system then computes a rotation matrix from three correction angles, in order to reharmonize the attitude of the reference frame of the display relative to the reference frame of the aircraft.
The main fault with this harmonization system based on the use of a boresight reference unit BRU is the inclusion of an additional item of equipment dedicated to just this realignment or harmonization function and a cost in terms of installation complexity, an additional bulk and weight that can be restrictive, in particular for small civilian aircraft. This BRU equipment item has to be powered through electrical wiring and installed in a robust manner. This BRU equipment item requires a lengthy harmonization procedure when it is installed with an additional error entry. A risk of misalignment through movement is possible for example upon installation or during a maintenance operation.
Furthermore, the exact parameters of orientation of this boresight reference unit BRU on the bearer, i.e. the bearing structure of the aircraft, has to be also introduced into the helmet-mounted display system HMD, and the BRU unit has to then always remain perfectly fixed relative to the bearer. Now, the current mechanical technologies do not make it possible to guarantee a mounting of the BRU unit in the cockpit without a risk of variations over time. Indeed, the vibratory environment, the interventions of the pilot and of the maintenance operators in particular can provoke slight rotations or movements of the boresight reference unit BRU, which results in the introduction of an error on the line of sight that cannot be compensated and, in many cases, that cannot be detected, and therefore the prevention of any subsequent reharmonization.
A first technical problem is how to provide a head-worn display system and a harmonization method which makes it possible to realign the symbology on the outside world when the head-up display or viewing system HWD/HMD has a mechanism for releasing and re-engaging the display in the field of view of the pilot, a source of misalignment, and to avoid the use of a calibration landmark installed inside the cockpit, also a source of error.
A second technical problem is how to more accurately determine the relative orientation M01 between the display D0 and the mobile tracking element D2 of the head posture detection subsystem DDP when the head-up display system HWD/HMD has a mechanism for releasing and re-engaging the display in the field of view of the pilot.
A third technical problem is how to correct the orientation of the aircraft supplied by its inertial station relative to the Earth, in particular for the heading whose value is generally not known with sufficient accuracy for a conformal display.